For a long time there has been increasing interest in power coatings which produce a matte surface. The reason for this is primarily a practical one: glossy surfaces require much more cleaning than non-gloss surfaces. Further, for safety reasons and reasons of occupational health, it may be desirable to avoid highly reflective surfaces.
The simplest principle bearing on the production of a matte surface is the incorporation and admixture of a filler into a powder coating to modify the gloss characteristics of the resultant coating. Generally, a filler such as chalk, finely divided silicon dioxide, barium sulfate, and/or an incompatible material such as a wax or cellulose derivative is added in a greater or lesser amount to the varnish depending on the degree of matting effect desired. However, such an admixture has a deleterious effect on the technical properties of the resultant coated film.
At the beginning of the 1970s, the development of powder coating compositions was initiated in which eventually a variation in reactivities was employed to produce a matte effect.
It is known from Dutch Pat. No. 68 06 930 that a matte effect can be produced by hardening an epoxy resin together with sulfamic acid and at least 2% of trimellitic anhydride.
German OS No. 21 47 653 describes a powder coating mixture which yields a matte effect, which mixture is produced by mechanically mixing at least two powder coating compositions, each comprising a different epoxy-resin-and-hardener system with a melting range which differs from that of the other(s).
Further, the process of Ger. OS No. 22 47 779 starts from a mixture of two powder coating compositions which differ from each other with respect to the presence or absence of a hardening accelerator.
Ger. Pat. No. 23 24 696 discloses a process for manufacturing coatings based on epoxy resin(s), said coating yielding matte surfaces. The coatings employ a special hardener, namely the salt of cyclic amidines with particular carboxylic acids. Actually, of the processes mentioned hereinabove, only this process is economically competitive. This is a consequence of the outstanding technical properties of the resulting coating. Meanwhile, the process has undergone improvements (see Ger. OS No. 30 26 455).
The epoxy resin systems disclosed in the prior art are not suitable for producing coatings which have substantial weather stability and light-resistance however. Materials which are particularly suitable for such areas of application are powder coating compositions based on polyesters, and .epsilon.-caprolactam-blocked aliphatic polyisocyanates. Thus, e.g., Ger. OS No. 21 05 777 describes powder coating compositions comprised of hydroxyl group containing polyesters and .epsilon.-caprolactam-blocked isophorone diisocyanate (IPDI) or adducts of the latter with polyols.
Ger. OS No. 29 29 150 describes powder coating compositions based on hydroxyl group containing polyesters melting between 40.degree. and 120.degree. C., with a proportion of .epsilon.-caprolactam-blocked IPDI-adducts. Among the suitable IPDI-adducts proposed in addition to the reaction products with polyols and water is trimerized IPDI, alone or in a mixture with monomeric IPDI. As is evident from the examples of the Ger. OS No. 29 29 150 powder coating compositions hardened by means of .epsilon.-caprolactam-blocked IPDI-derivatives always give coatings with glossy surfaces. In light of the difficulties involved in preparing powdered coating compositions of polyesters and blocked polyisocyanates which have a matte appearance, a need continues to exist for such resin compositions which exhibit an improved matte surface.